Electroabsorption modulators have been widely used to modulate light in optical communications and other applications. For example, an electroabsorption modulator can modulate the light generated by a light source, such as a continuous light source. The electroabsorption modulator modulates light by either allowing or preventing incident light to pass through the electroabsorption modulator. The modulated light output from the electroabsorption modulator has an ON state and an OFF state. In the ON state, the light passes through the electroabsorption modulator and is emitted from the output of the electroabsorption modulator. In the OFF state, the light is absorbed by the electroabsorption modulator, and no light is emitted at the output of the electroabsorption modulator. As used in this disclosure, the term light means electromagnetic radiation ranging in wavelength from short ultra-violet to long infra-red.
One type of electroabsorption modulator includes a p-i-n junction structure composed of an active layer of intrinsic semiconductor material (intrinsic layer) sandwiched between a cladding layer of p-type semiconductor material (p-type cladding layer) and a cladding layer of n-type semiconductor material (n-type cladding layer). This type of electroabsorption modulator modulates the light by switching between a transmissive mode and an absorptive mode. In the transmissive mode, the band gap of the semiconductor material of the active layer is greater than the photon energy of the light. The photons of the light therefore have insufficient energy to generate electron-hole pairs in the active layer, and absorption of the light by the active layer is negligible. Consequently, light from the light source passes through the active layer and is output by the electroabsorption modulator. In the transmissive mode of the electroabsorption modulator, the modulated light is in its ON state.
In the absorptive mode of the electroabsorption modulator, the band gap of the material of the active layer is effectively reduced below the photon energy of the light. The photons of the light now have sufficient energy to generate electron-hole pairs in the semiconductor material of the active layer. As a result, absorption of the light by the semiconductor material of the active layer is substantial. When sufficient electron-hole pairs are generated, all of the light is absorbed in the active layer, and none of the light passes through the active layer and is output by the electroabsorption modulator. In the transmissive mode of the electroabsorption modulator, the modulated light is in its OFF state.
Two of the main parameters that characterize the light modulation performance of an electroabsorption modulator are extinction ratio and speed of modulation. The extinction ratio is the ratio of the maximum optical power output by the electroabsorption modulator to the minimum optical power output by the electroabsorption modulator. A higher extinction ratio is typically the result of a higher absorption of the light through the creation of more electron-hole pairs in the active layer. Speed of modulation indicates the time required for the electroabsorption modulator to modulate the light from the ON state to the OFF state. Electroabsorption modulators with high extinction ratios and high speeds of modulation are typically desired.
The active layer of an electroabsorption modulator is typically structured to include a quantum well structure that defines one or more quantum wells. Light is absorbed in the active layer of the electroabsorption modulator when an electric field is applied to the quantum wells in a direction orthogonal to the layers of the quantum well structure. Applying the electric field changes the effective band gap energy of the quantum well structure through the quantum-confined Stark effect.
Electroabsorption modulators absorb incident light when a reverse bias is applied to the p-i-n junction structure. Because a negligible current flows when the reverse bias is applied, the speed of modulation of the electroabsorption modulator is limited by the time taken to apply the reverse-bias voltage. Consequently, the speed of modulation of the electroabsorption modulator is limited by the capacitance of the electroabsorption modulator and the resistance and inductance of the circuitry that supplies the reverse bias voltage to the electroabsorption modulator.
In conventional electroabsorption modulators, a tradeoff has to be made between the extinction ratio and the speed of modulation. High-speed operation of the electroabsorption modulator requires that the p-i-n junction structure have a very low capacitance. To reduce the capacitance, the thickness of the active layer is increased, to about 300 nanometers, for example. However, increasing the thickness of the intrinsic layer reduces the extinction ratio, which is typically undesirable. The extinction ratio is reduced because the strength of the electric field applied across the quantum wells is reduced as a result of a given reverse bias voltage being dropped across a thicker intrinsic layer. As a result, the quantum-confined Stark shift and the light absorption are both reduced. Consequently, a low-capacitance electroabsorption modulator requires an increased reverse bias voltage to achieve a desired extinction ratio.
However, increasing the reverse bias voltage increases the width of the depletion region that exists in the p-i-n junction structure when the reverse bias voltage is applied. The depletion region exists in the active layer and additionally extends into the cladding layers. The p-type and n-type cladding layers are typically heavily doped to reduce the extent to which the depletion region extends into them with the purpose of reducing the effective width of the depletion region. However, the extent to which there is extension of the depletion region into the cladding layers reduces the strength of electric field across the quantum wells of the electroabsorption modulator and, hence, reduces the extinction ratio of the electroabsorption modulator. Moreover, the need for a higher reverse bias voltage to achieve a desired extinction ratio conflicts with the current trend in high-speed drive circuit design, which is towards lower supply voltages and, hence, lower voltage swings.